Windchill 10.2 requires Java 7 for its applets as well as for running the server.
While the applets themselves have mostly not evolved much as of late, some of the classes they use make use of Java 7 language features and/or direct (non-reflective) use of Java 7 APIs. Moreover, all of Windchill 10.2 (apart from the wmx.jar bootstrapping jar) is compiled with a target version of 1.7 so that all classes therein can use Java 7 language features and APIs.
As for 10.2 M030, PTC is working to replace as many applets as possible in M030. This will not, however, be all applets. You are right in surmising that the remaining applets are likely to evolve much further at this point, though.
Publicly available security updates to Java 6 ended some time ago, so unless you're paying Oracle for support and updates, that means you probably want to move to Java 7 for security updates anyway.